Titanium is extracted from ore by the production of titanium tetrachloride and its subsequent reduction to titanium by reaction with either magnesium or sodium. Electrolytic reduction may also be used. The reduction product is a porous, spongy material, which is referred to as titanium sponge. Titanium sponge is typically converted to titanium alloy products, including mill products, by a series of consumable electrode melting operations. Initially, the titanium sponge with either alloy particles mixed therewith or unalloyed in particle form is typically pressed to compact the particles to form compacts or brickettes that are joined as by welding to produce an electrode. Alloying elements may be introduced in admixture with the sponge particles in desired amounts for producing titanium-base alloys. The electrode is vacuum arc melted to produce an ingot which typically is then re-melted once or twice to produce a final ingot. The ingot resulting from either double or triple vacuum arc melting is then typically processed by a series of forging and annealing steps to form from the ingot products such as a billet or shaped products that are machined to desired final product configurations.
During the production of titanium sponge typically reaction with the ambient atmosphere may cause a reaction of particles of the titanium sponge with air to form nitride or oxynitride particles, which ultimately can result in defects known as low density inclusions in the ingot and products made therefrom. These nitrogen-rich particles are highly refractory and thus do not melt during the conventional vacuum arc melting steps incident to the production of a titanium ingot. Typically, the nitride or oxynitride particles slowly dissolve during the vacuum arc melting operation. As the particles dissolve slowly, they tend to sink to the bottom of the molten metal pool formed during vacuum arc melting. If not completely dissolved when they sink to the bottom of the pool the titanium solidifies above these undissolved particles to entrap them in solid titanium. This retards dissolving of the particles, because the dissolution of the particles is by a solid state reaction process. Consequently, these particles result in defects in the form of highly refractory, hard, porous particles which are contained within the solidified ingot. These defects are known as low density inclusions. They are termed "low density" inclusions because of their apparent low density when detected by conventional X-ray inspection. During this inspection, their porous nature allows greater X-ray penetration than the remainder of the product so that they appear as a lower density material. In contrast, inclusions of refractory metals, such as tungsten and molybdenum, allow less X-ray penetration and consequently are termed high density inclusions.
During subsequent reduction of the titanium ingot as by forging and/or rolling to the desired shapes, such as billet, bar and plate, the low-density inclusions remain within these products. For this reason, the products are conventionally subjected to ultrasonic and/or X-ray inspection for detection of low-density inclusions, and low-density inclusions so detected are removed by removing the sections of the product containing them. Inevitably, however, some low-density inclusions go undetected and may find their way into final products. Low-density inclusions in final products provide sites for crack initiation, particularly when the products are subjected to stress during service, to result in failure of these final products.